“We expect to have well over 100 preschool, pre-K and early-learning teachers and staff there,” said Heather Weiner, spokeswoman for Yes for Early Success, a coalition funded primarily by Service Employees International Union (SEIU) Local 925 and the American Federation of Teachers-Washington (AFT).

Both unions endorsed former Seattle Mayor Mike McGinn in the election last fall.

In the midst of McGinn’s re-election campaign, the city issued directives telling child-care providers who contract with the city that they had to meet with union representatives or lose city funding — a move vigorously opposed by the YMCA of Greater Seattle and other child-care providers.

McGinn lost to state Sen. Ed Murray and that idea wasn’t revived.

But the unions have remained engaged in discussion of the city’s plan, which would make high-quality preschool free for Seattle families earning no more than twice the federal poverty rate, which amounts to $47,700 for a family of four. Other families would pay on a sliding scale.

“We are trying to keep all of our options open, and one of the ways we’re doing that is by preparing for a ballot initiative if necessary,” Weiner said.

City Council President Tim Burgess, who is leading the universal preschool effort, said a competing measure on the ballot could endanger the whole plan.

“If that happens, that would be very destructive,” Burgess said. “There has to be very strong clarity or else voters get confused, or they perceive conflict, and it makes it very difficult to win passage,” Burgess said.

Child-care and prekindergarten teachers in King County make, on average, $13.93 an hour, and assistant teachers make $11.35, Weiner said. The ballot measure would set a $15 minimum wage.

They’ll get no quarrel with Burgess over the pay issue.

“Qualified preschool teachers should be paid just like teachers in the K-12 system,” Burgess said. He said the council will have to decide how to pay assistant teachers enough to stay in the preschool field.

Seattle kindergarten teachers on average make about $33 an hour in base salary, plus other categories of pay that can represent a third or more of their compensation, according to the district.

The coalition’s proposed ballot measure also would require child-care teachers and staff to obtain training and certification through a “Professional Development Institute” jointly operated by the city and a single provider hired by the city, which could be a union, but not necessarily, Weiner said.

“There’s nothing in here that has any kind of mandatory or even incentivized unionization,” Weiner said. “What is important is that the teachers, and the center directors and the providers have a seat at the table when you’re setting these standards and determining what training is necessary.”

Burgess said many different groups are part of that conversation.

“We are focused on providing adequate support for teacher education and coaching and mentoring of teachers,” Burgess said. “There are lots of people who have interest in that area, not just SEIU or AFT.”